Methods of forming nuclear
fuel bodies



United States Patent 3,344,211 METHODS OF FORMING NUCLEAR FUEL BUDIESGeoffrey Brian Redding, Weymouth, and David Norman Lamb, Poole, England,assignors to United Kingdom Atomic Energy Authority, London, England NoDrawing. Filed Oct. 20, 1965, Ser. No. 499,040 Claims priority,application Great Britain, Oct. 30, 1964, 44,460/64 8 Claims. (Cl.264-.5)

ABSTRACT OF THE DISCLOSURE Particles, particularly nuclear fuelparticles, are coated by tumbling in a drum with powder grains of abindertreated coating material. The binder is softened by contacting thepowder grains with a binder solvent as the grains enter the drum, sothat the particles acquire an outer coating of the grains by asnowballing effect. The coating allows of the coated particles beingformed into compacts by pressing and heating to set the binder, withoutthe need for adding further filler or matrix material.

This invention relates to a method of treating particles.

The invention is particularly applicable to methods of forming nuclearfuel bodies which consist of a dispersion of particles containingnuclear fuel in a matrix. Several methods of making these fuel bodieshave been proposed hitherto. For instance it has been proposed to mixthe fuel particles (each enclosed in a fission product retainingcoating) with a carbonaceous powder material and a binder and to pressthe mixture in a die whilst heating to polymerise the binder. Anothermethod has been to fill in the spaces between a loose mass of particlesby depositing carbon pyrolytically from the gas phase. A third method,and one with which the invention is directly concerned, is that ofapplying an outer coating to the particles thick enough to allow a batchof particles so coated to be moulded under pressure in the green state.The advantage of the last mentioned method is that the difficultieswhich arise in trying to mix dense fuel particles with light powderedgraphite are avoided, the total amount of carbon in the finished compactcan be closely controlled and a considerable uniformity of distributionof particles can be easily achieved.

It is one object of the invention to provide an improved method ofapplying an outer coating to fuel particles being a coating which allowsof the particles being formed into compacts without the need for addingfurther filler or matrix material.

According to one aspect of the invention a method of treating particlesresides in tumbling the particles in a drum into which powder grainswhich have been previously treated with a binder are introduced, andsoftening the binder by contacting the grains with a solvent for thebinder as the grains enter the drum such that the particles acquire anouter coating of the powder grain by a snowballing effect.

According to the invention in another aspect a process for formingcompacts containing nuclear fuel particles dispersed in a matrixmaterial resides in applying to each of the particles an outer layer ofmatrix forming material by a method which resides in providing a supplyof matrix forming material in powder form whose grains have been treatedwith a binder, tumbling the particles in a drum into which the powdergrains of the matrix forming material are introduced as a free flowingpowder, contacting the powder grains as they enter the drum with a mistof a volatile solvent for the binder, such that the particles acquire anouter coating of the powder grains by a snowballing effect, terminatingthe tumbling after the required amount of matrix forming material hasbeen introduced and then forming a batch or mass of the particles socoated into compacts by pressing and heating to set the binder.

Preferably the particles are heated in vacuum to evaporate any solventremaining after they are removed from the drum and prior to heating inthe press.

The particles used in the process may have been precoated with a fissionproduct retaining layer or layers before being tumbled with matrixforming material. Alternatively or additionally the whole compact may beenclosed in a fission product retaining coating layer or container. Thematrix forming material is preferably a carbonaceous material such asgraphite and the nuclear fuel particles may be carbides of nuclear fuelmetals, pre-coated with a fission product retaining layer of pyrolyticcarbon.

One process embodying the invention in which the preferred carbonaceousmatrix is employed will now be described.

As a preliminary process step, a supply of powdered matrix formingmaterial whose powder grains carry resin binder material was prepared asfollows:

A 2,000 kilogram batch of calcined petroleum coke (Shell H- reformerresidue) was crushed, sieved to yield the particle size below 251microns. The crushed coke was then micronised and the fraction below 50microns was graphitised at 2600 C., the course fraction being reground.

The graphitised grains represented a 1500 kg. yield and these were mixedwith a phenol formaldehyde resin dissolved insolvent. After mixing forhalf an hour, the material was removed from the mixer, the solventhaving dried off. The product was then reduced to crumbs and sieved toselect the particle fraction below 76 microns. The final yield whichconstituted the matrix forming material was 1100 kilograms, having a tapdensity of 1.16 grams/cc. This material was used to form compacts in themanner now described.

A batch of nuclear fuel particles pro-coated with pyrolytic carbon as afission product retaining layer was placed in a rotary drum mounted forrotation on an axis which is inclined at a small angle to the horizontaland having one open end for the introduction of process materials. Thedrum was rotated on its axis at a speed of about 30 r.p.m.

A supply of graphite grains of matrix forming material and a supply ofbinder solvent methylated spirit were mounted above the drum.

Suitable feed pipes were arranged to communicate the supply of grainsand solvent with the interior of the drum, the pipes terminating withinthe drum at adjacent positions so that the grains and a mist of thesolvent contacted one another after these material had left their feedpipes and before they impinged on the drum surface. A rate of solventinjection of 10 cc. per hour was found adequate where the graphitegrain-s were introduced at 600 grams per hour through an orifice 2millimetres in diameter.

As the solvent mist made contact with the graphite grains, the resincontent of the latter becomes softened and so the grains tended to stickto the surfaces of the tumbling particles which grow in size owing tothe snowball effect. An outer coating is thus applied to each particlewhich is spherical in shape due to the tumbling motion of theparticlesBy proper adjustment of the graphite and solvent feed asubstantially even distribution of graphite amongst the charge ofparticles was achieved resulting in a substantially uniform coating oneach particle.

The tumbling and graphite introduction was maintained for about fourhours and then stopped, by which time some 2,500 grams of graphitegrains had been gathered up by the 500 grams of particles. The particleswere then removed from the drum and placed in a furnace where they wereheated in vacuum up to 50 C. for half an hour to evaporate any solventremaining.

The particles, which had been easily handable without fear of crushingor disintegration since they were removed from the drum, were thendivided into batches suitable for forming compacts.

This was done by weighing out the quantity previously found to beappropriate for filling a die, which is capable of being heatedelectrically. The correct quantity was then placed in the die which wasthen closed and placed in a press. The filling was then subjected to1,000 p.s.i. whilst it was simultaneously heated to 800 C. At thistemperature the phenol formaldehyde binder polymerised and the outercoating around each layer deformed to fill any voids between theparticles or between the particles and the inner surface of the die. Thecompact was then removed from the die.

By following the above procedure, not only are the correct proportionsof filler and particles in the compact ensured, but also the correctamount and distribution of the binder.

We claim:

1. A method of applying to nuclear fuel particles an outer coating whichresides in tumbling the fuel particles in a drum into which powdergrains of a matrix material which have been previously treated with abinder are introduced, and softening the binder by contacting the powdergrains with a solvent for the binder as the grains enter the drum suchthat the fuel particles acquire an outer coating of the powder grains bya snowballing effect, which coating allows of the fuel particles beingformed into compacts by pressing and heating without the need for addingfurther filler or matrix material.

2. A method as claimed in claim 1 wherein the matrix forming material isa free flowing powder whose grains have been treated with and carry abinder, and said solvent is a volatile solvent for the binder.

3. A method as claimed in claim 2 including the step of forming aselected mass of the fuel particles so coated into compacts by pressingand heating to set the binder.

4. A method as claimed in claim 1 in which the powder grains are ofcarbonaceous material and have been treated by mixing such grains below50 microns with a liquid resin binder, allowing the solvent toevaporate, forming the product into crumbs, and selecting a suitablesize fraction.

5. A method of forming compacts containing nuclear fuel particlesdispersed in a matrix material comprising applying to each of theparticles an outer layer of matrix forming material by a method whichcomprises providing a supply of matrix forming material in powder formwhose grains have been treated with and carry a binder, tumbling thefuel particles in a drum into which the powder grains of the matrixforming material is introduced as a free flowing powder, contacting thepowder grains as they enter the drum with a mist of a volatile solventfor the binder, such that the particles acquire an outer coating of thepowder grains by a snowballing effect, terminating the tumbling afterthe required amount of matrix forming material has been introduced andthen forming a selected mass of the patricles so coated into compacts bypressing and heating to set the binder.

6. A method of coating nuclear fuel particles which resides in mixingthe grains of a matrix forming material with a binder dissolved in asolvent, drying off the solvent, reducing the resultant cake to crumbs,sieving the crumbs to select a powder fraction below a given size,introducing the fuel particles to be coated into a revolving drum,introducing the powder into the revolving drum, tumbling the fuelparticles to be coated in the drum and contacting the binder-treatedpowder grains of coating material with a binder solvent mist immediatelybefore they enter the drum so that they become susceptible to adheringto and coating the fuel particles tumbled in the drum, thus formingcoated fuel particles which can be formed into compacts by pressing andheating without the need for further filler or matrix material.

7. A method as claimed in claim 5 in which the matrix forming materialis petroleum coke grains coated with phenol formaldehyde.

8. A method of forming compacts containing particles of a first ceramicmaterial dispersed in a second ceramic matrix material comprisingapplying to each of the particles of the first ceramic material an outerlayer of the second ceramic matrix forming material by a method whichcomprises providing a supply of the second matrix forming material inpowder form whose grains have been treated with and carry a binder,tumbling the particles of the first ceramic material in a drum intowhich the powder grains of the matrix forming material are introduced asa free flowing powder, contacting the powder grains as they enter thedrum with a mist of a volatile solvent for the binder, such that theparticles of the first ceramic material acquire an outer coating of thepowder grains by a snowballing effect, terminating the tumbling afterthe required amount of matrix forming material has been introduced andthen forming a selected mass of the particles so coated into compacts bypressing and heating to set the binder.

References Cited UNITED STATES PATENTS 1,105,864 8/1914 Waddell 26433,173,973 3/1965 Brockway 264.5 3,211,812 10/1965 Johnson et al. 264.5

OTHER REFERENCES AEC Documents: (1) BMI-l321, 1959, pp. 1-3. (2)BMI-1579, 1963, pp. C-l to C4.

L. DEWAYNE RUTLEDGE, Primary Examiner.

1. A METHOD OF APPLYING TO NUCLEAR FUEL PARTICLES A OUTER COATING WHICHRESIDES IN TUMBLING THE FUEL PARTICLES IN A DRUM INTO WHICH POWDERGRAINS OF A MATRIX MATERIAL WHICH HAVE PREVIOUSLY TREATED WITH A BINDERARE INTODUCED, AND SOFTENING THE BINDER BY CONTACTING THE POWDER GRAINSWITH A SOLVENT FOR THE BINDER AS THE GRAINS ENTER THE DRUM SUCH THAT THEFUEL PARTICLES ACQUIRE AN OUTER COATING OF THE POWDER GRAINS BY ASNOWBALLING EFFECT, WHICH COATING ALLOWS OF THE FUEL PARTICLES BEINGFORMED INTO COMPACTS BY PRESSING HEATING WITHOUT THE NEED FOR ADDINGFURTHER FILLER OR MATRIX MATERIAL.
 5. A METHOD OF FORMING COMPACTSCONTAINING NUCLEAR FUEL PARTICLES DISPERSED IN A MATRIX MATERIALCOMPRISING APPLYING TO EACH OF THE PARTICLES AN OUTER LAYER OF MATRIXFORMING MATERIAL BY A METHOD OF WHICH COMPRISES PROVIDING A SUPPLY OFMATRIX FORMING MATERIAL IN POWDER FORM WHOSE GRAINS HAVE BEEN TREATEDWITH AND CARRY A BINDER, TUMBLING THE FUEL PARTICLES IN A DRUM INTOWHICH THE POWDER GRAINS OF THE MATRIX FORMING MATERIAL IS INTRODUCED ASA FREE FLOWING POWDER, CONTACTING THE POWDER GRAINS AS THEY ENTER THEDRUM WITH A MIST OF A VOLATILE SOLVENT FOR THE BINDER, SUCH THAT THEPARTICLES ACQUIRE AN OUTER COATING OF THE POWDER GRAINS BY A SNOWBALLINGEFFECT, TERMINATING THE TUMBLING AFTER THE REQUIRED AMOUNT OF MATRIXFORMING MATERIAL HAS BEEN INTRODUCED AND THEN FORMING A SELECTED MASS OFTHE PARTICLES SO COATED INTO COMPACTS BY PRESSING AND HEATING TO SET THEBINDER.